With few exceptions, all prior arts teaching the construction and operation of electronic filters for conditioning electrical signals in the vortex flowmeters employ the well known and routinely practiced feedback method to control the switching of a plurality of filters included in a parallel bank of filters. In other words, these prior arts measure the frequency of the alternating electrical signal generated by the vortex shedding in the flowmeter at the output end of the parallel bank of filters and generates a dc voltage from the measured value of the frequency of the alternating electrical signals, which dc voltage is fed back to switch on one of the filters that selectively transmits the alternating electrical signal oscillating at the measured frequency. Generally, the above-described feedback method controlling the switching of individual filters included in the parallel bank of filters does not work in conditioning the alternating electrical signal of very low amplitude generated by the vortex shedding at very low velocities of fluid flow, because the parallel bank of filters controlled by the feedback command signal locks on the noise signal created by mechanical vibrations of the vortex flowmeter rather than on the very weak vortex signal. The parent patent application Ser. No. 07/848,291 discloses a method that overcomes the problem and difficulty existing with the prior arts, in which method the switching of individual filters included in a parallel bank of filters is controlled by a command signal generated by the frequency of the alternating electrical signal measured at the input end of the parallel bank of filters instead of the output end thereof. It has been discovered that the method controlling the switching of the individual filters constituting the parallel bank of filters by generating a command signal from the frequency of the alternating electrical signal taken at the input end of the parallel bank of filters works best, when the alternating electrical signal put into the parallel bank of filters is preconditioned by another parallel bank of filters including at least two parallel individual filters.